This is some of the worst advice I've seen related to this fight.
Here is how you should do it, and it worked for us after struggling on that fight.
Stack Sul & Kazrajin (Melee Boss?) together and nuke Sul down with all cooldowns available until Kazrajin is around 50% Energy. Switch to him, and nuke him so he doesn't start pulsating raid damage. Continue to nuke Sul whenever possible, and leave one good DPS on him to finish him off.
Leave the priestess out of the fight the whole time while she isn't empowered. Once empowered, bring her into the middle and AoE Cleave her down while nuking Sul still. By this time, Sul should be nearing death, and the Priestess should be nearing full energy. Switch and nuke her before she starts the AoE raid damage.
Then move her back out to avoid an instant heal (have the tank and a DPS interrupt her so she can be moved)
Getting Sul down A.S.A.P. is your priority. Once he is down, the fight becomes a joke because the healers haven't got to panic about the Sandstorm or the adds it brings. Have your team reassess their Talents, Spec, Enchantments & Gems. Try using something like
www.askmrrobot.com for a starting point.